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Poster: librivoxbooks Date: Feb 6, 2013 4:12am
Forum: audio_bookspoetry Subject: Reduction in volume in derived files

We have noticed that 128 kbps audio files uploaded at a certain volume in dB lose approximately 3-4 dB in the conversion process to 64 kbps and OGG during derivation. Our own experiments do not replicate this effect, and we wonder if there is some element in the derivation process that is causing this to happen. If so, is there any way that it can be changed so that the volume remains reasonably consistent across the different formats?

Example: http://archive.org/details/humanmachine_rg_librivox

http://archive.org/download/humanmachine_rg_librivox/humanmachine_12_bennett.mp3 128 kbps: 89.5 dB
http://archive.org/download/humanmachine_rg_librivox/humanmachine_12_bennett_64kb.mp3 64 kbps: 86.4 dB
http://archive.org/download/humanmachine_rg_librivox/humanmachine_12_bennett.ogg OGG: 86 dB

Interestingly, the mono file uploaded also becomes stereo when converted to OGG.

A reply from one of the tech team would be most appreciated.

Ruth

This post was modified by librivoxbooks on 2013-02-06 12:12:30

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Poster: Jeff Kaplan Date: Feb 6, 2013 9:35am
Forum: audio_bookspoetry Subject: Re: Reduction in volume in derived files

what are you using to analyze the decibel level?

Reply [edit]

Poster: librivoxbooks Date: Feb 8, 2013 4:39am
Forum: audio_bookspoetry Subject: Re: Reduction in volume in derived files

MP3Gain and/or our own Checker program. I think our Checker program uses a similar algorithm to MP3Gain, though.

Also the volume is clearly different in Audacity: you can amplify the 128kbps file by 1.8 dB to 0 dBFS, while the 64 kbps and OGG can be amplified by over 5 dB.

I chose those files as an example as they have a very even volume throughout nearly all the recording. Not allowing 0dBFS to be exceeded in Audacity means that there is a difference between these actual measurements and the measurement in MP3Gain (which is an average of the whole recording), but relative to each other, the 128 kbps, 64kbps MP3s and OGG still show the >3 dB difference.

Ruth

This post was modified by librivoxbooks on 2013-02-06 21:32:13

This post was modified by librivoxbooks on 2013-02-08 12:39:47